[volt-nuts] Fluke 731A Transfer Standard and Fluke Wire Wound Resistors

J. L. Trantham jltran at att.net
Sun Sep 16 20:36:38 UTC 2012

I have been following the discussion about the LM399 heater, noting mention
of the Fluke Wire Wound resistors used often in Fluke precision equipment.


I have a 731A that I have been resuscitating and have found that the R8A R8B
wire wound resistor is failing.  Initially, the source was very unstable
then went to about 8 VDC.  This led me to an open R8B.  I was able to
‘reconnect’ the broken ends, loosing about half a turn of wire.  Then,
again, unstable and went to about 13 VDC.  This led me to an open R8A.
Again, able to reconnect, and, again, loosing about ½ turn.


It appears that the wire is corroded with corrosion leading to failure.
This brings up several questions.


1.	What are the benefits of these wire wound resistors?  Tempco?
Ability to construct precise resistance?
2.	Is there a source of the resistance wire used in these resistors so
that the resistor can be reconstructed?  I think the resistance is in the 30
to 40 ohms per foot range.  Something like 36 to 38 gauge Nichrome 60 would
work but it has to be insulated.
3.	Would it be better to look for a collection of commercial resistors
to ‘replace’ R8A, R8B?  If so, what?  The manual states that IC2, the
voltage reference, R6, R7, R9, and R10, are ‘factory selected’ and all
appear to be OK.
4.	Would it be better to completely abandon the voltage reference
section of the 731A in favor of a ‘new’ (perhaps LM399, LTZ1000A, etc.,
based) reference to generate 10 VDC and still use the output divider of the
731A to generate the remaining voltages?


The 731A manual and schematic are here:




R8A is the ‘high’ side of a voltage divider with R8B, R9, R10 and R11 (a 10
ohm, 20 turn pot) forming the ‘low’ side, dividing 10 VDC to feed the
voltage reference, IC2, a DH80417B.  The op amp, IC1, used to generate the
10 VDC is an LM301AH.


R8A measures about 5903 ohms, R8B measures about 11558 ohms, R9 measures
about 78.02 ohms, and R10 measures about 399.8 ohms.


Thanks in advance for any and all information and suggestions.



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